Ultrasonically Vibrated Die Rings

ABSTRACT

An ultrasonically vibrated die comprises a generally cylindrical die ring supported by a coaxial resonant mounting tube. The die ring is vibrated in a radial bending mode of vibration, in which an end surface of the die ring oscillates between a concave and a convex state. The mounting tube joins the end surface of the die ring at a radius R where the amplitude of the oscillation of the end surface is at a minimum, in order to reduce transmission of the vibration into the mounting tube.

TECHNICAL FIELD

The invention relates to an apparatus for forming metal workpieces bydriving the workpieces into a die. It has particular application toannular workpieces that commonly have circular symmetry about the axisof movement, whereby the forming process changes the longitudinalprofile of the workpiece, for example to form a neck of reduced radiusand predetermined shape.

BACKGROUND OF THE INVENTION

It has long been known to change the longitudinal profile of an annularor tubular workpiece by driving the workpiece along its axis of symmetryinto a die of suitable shape to form the desired profile—or into asuccession of dies that are respectively shaped to create the desiredprofile in a sequence of smaller steps. It is also known that vibratingthe die at ultrasonic frequencies can assist the forming process byreducing the friction between the die and the workpiece and/or byenhancing the way the working surface of the die acts on the workpieceto deform it.

U.S. Pat. No. 4,854,149 (Porucznik et al.) illustrates examples of suchan ultrasonically-assisted forming process. The end of the workpiece tobe formed is inserted coaxially into the profiled aperture of a diering. A transducer is attached to the die ring at a location on itscircumference and delivers ultrasonic energy into the die ring. Thetransducer vibrates along its own longitudinal axis, which is alignedwith a radius of the die ring. The radial application of ultrasonicvibrations to the die ring induces resonant modes of vibration,depending on the shape and material of the die ring and the frequencyapplied. The die ring is mounted on a forming machine via a mountingtube that is coaxial with the die ring.

The die ring needs to be mounted firmly enough to withstand the highforces exerted on it during the forming of a metal workpiece, whileallowing it to vibrate as freely as possible at the applied frequency.It is desirable to minimize the transmission of vibrations from the diering into the mounting tube, both because this causes energy to be lostfrom the die ring and because it may interfere with the desired mode ofvibration of the die ring.

U.S. Pat. No. 5,095,733 (also Porucznik et al.) discloses and classifiesvarious possible resonant modes of a ring-shaped die. It teaches thatthe preferred mode is a pure radial mode termed “RO”, in which the diering expands and contracts radially, centred on the axis of the ring, asthe axial length respectively contracts and expands to a lesser extent.

The present inventors have found that the pure radial mode RO cannotgenerally be achieved at suitable frequencies and within the typicalspace constraints of a die in a forming machine. However, the die ringcan readily be induced to vibrate in a “radial bending” mode termed“RB0”, which is schematically illustrated in FIGS. 1A to 1C. FIG. 1Ashows a simple, hollow cylinder in its resting state. Because theharmonic number is zero, this mode continues to display circularsymmetry about the axis of the ring, whereby in ideal circumstances thecontact between the working surface of the die ring and the workpiece issynchronous around any given circumference. The expansion andcontraction are also substantially synchronous along the axis of the diering. However, the amplitude of the vibration is not uniform along theaxis. In particular, the component oscillates between an hourglass shape(FIG. 1B) and a barrel shape (FIG. 1C) over a cycle of the vibration,passing through approximately its original cylindrical configuration(FIG. 1A) at the midpoint between each of these two extremes. It can beseen that in the “hourglass” configuration of FIG. 1B, the annular endsurface of the component bulges outwards in a convex cone, while in the“barrel” configuration of FIG. 1C, the annular end surface of thecomponent sinks inwards in a concave cone. Note that the shape of theend surface in these configurations is not necessarily a true cone, i.e.a plane containing the axis may intersect the end surface in a curvedline rather than a straight line.

SUMMARY OF THE INVENTION

The invention provides a die, comprising:

a generally cylindrical die ring comprising an end surface and having aradial bending (RB0) mode of vibration in which the end surfaceoscillates between a concave and a convex state; and

a mounting tube coaxial with the die ring and extending from the endsurface of the die ring;

characterized in that the mounting tube joins the end surface of the diering at a radius where the amplitude of the oscillation of the endsurface is at a minimum.

The invention also provides a method of operating a die that comprises agenerally cylindrical die ring having an end surface; and a mountingtube coaxial with the die ring and extending from the end surface. Themethod comprises vibrating the die ring in a radial bending (RB0) mode,in which the end surface of the die ring oscillates between a concaveand a convex state, characterized in that the minimum amplitude of theoscillation of the end surface occurs at a radius where the mountingtube joins the end surface.

By making the radius of the mounting tube join the end surface of thedie ring at a radius where the amplitude of the oscillation is at aminimum, the undesired transmission of vibrational energy from the diering into the mounting tube can be reduced. This is an unforeseenadvantage compared with the prior art suggestion that the die ringshould be vibrated in a pure radial mode RO, because in the RO mode allpoints on the die ring oscillate in phase and there does not exist aradius at which the amplitude of oscillation reaches a minimum.

THE DRAWINGS

FIGS. 1A to 1C are perspective views of a computer model of an annularcomponent undergoing vibration in radial bending mode RB0.

FIG. 1D is a schematic sectional view of the end wall of the componentof FIG. 1A, shown at the two extremes of its vibration.

FIGS. 2A and 2B are perspective views in different orientations of a diein accordance with the invention.

FIG. 3 is a longitudinal section of the die of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1D schematically shows the end wall of the component of FIG. 1A atthe two extremes of its vibration in the radial bending mode RB0. Dottedlines 30 show the component in its “hourglass” configuration,corresponding to FIG. 1B. Solid lines 32 show the component in its“barrel” configuration, corresponding to FIG. 1C. It can be seen thatthe movement of any point on the surface of the end wall between the twoextremes is principally in a direction parallel to the axis 34. A pointP on the radially outer part of the end surface moves between a greateraxial elongation in the barrel configuration and a smaller axialelongation in the hourglass configuration, while a point Q on theradially inner part of the end surface does the opposite, oscillating180° out of phase with the outer part. At a point in between, at anintermediate radius R, the amplitude of the oscillation of the endsurface must be at a minimum. In fact, the movement of the points on theend surface is not in general purely axial—there is also a radialcomponent—but it is still true that at an intermediate radius thereexists a circle of points on the end surface where the amplitude of theoscillation of the points is at a minimum.

The amplitude may be defined in various ways. Preferably, it is thestraight-line distance between the corresponding points at the twoextremes of the oscillation. Alternatively, the amplitude may bemeasured along the path that a point on the surface follows betweenthose two extremes. Another possibility is to measure only the componentof the movement parallel to the axis. If preferred, the amplitude may bedefined as one half of any of the aforementioned values, to conform tothe conventional definition for a waveform; this makes no difference toidentifying the radius at which the minimum value occurs.

FIGS. 2A, 2B and 3 illustrate a die 1 according to an embodiment of thepresent invention. The die 1 incorporates a die ring 2 that defines acentral axis 3. The die ring 2 is formed integrally with a resonantmounting tube 4. The mounting tube 4 is coaxial with the die ring 2 andextends axially from an end surface 5 of the die ring 2. Part way alongthe tube 4 is a radially projecting flange 6, which is used for mountingthe die 1 on a forming machine (not shown) to support the die ring inuse. As can be seen in FIG. 3, the section of the tube 4 between the diering 2 and the flange 6 is thin-walled so as to be relatively flexibleand to minimize the coupling of the vibration of the die ring 2 into thetube 4.

The die ring 2 has a central aperture 8 that opens to the axial endremote from the mounting tube 4. The interior wall of the aperture 8defines a working surface 10 that is profiled to form a tubularworkpiece (not shown) as it is driven into the aperture against theworking surface 10. The die ring 2 is vibrated ultrasonically to assistthe forming process.

The outer surface 12 of the die ring 2 is generally cylindrical. At onepoint on its circumference there is formed a planar surface, parallel tothe axis, that acts as an interface 14 for an ultrasonic transducer (notshown). The interface surface 14 has a threaded bore 16 in its centrefor receiving a stud (not shown) that is used to secure the transducer.

The shape and material of the die ring 2 are chosen such that, when anultrasonic transducer is coupled to the interface 14 and introducesenergy at a predetermined frequency, the die ring 2 vibrates in thepreviously described radial bending mode RB0. During this vibration, theend surface 5 oscillates between a convex and a concave configuration asillustrated in FIG. 1D. The radius R of the mounting tube 4 where itjoins the end surface 5 is equal to the radius where the amplitude ofthis oscillation of the end surface 5 is at a minimum. More precisely,the circle of points on the end surface where the oscillation is aminimum lies within the thickness of the wall of the mounting tube.

Because the mounting tube 4 is thin-walled and flexible, to a firstapproximation the vibration modes of the die ring 2 can be consideredindependently from those of the mounting tube 4. The mounting tube 4joins the end surface 5 of the die ring 2 where the amplitude ofvibration is at a minimum, so it is desirable to design the mountingtube 4 such that at the operating frequency the vibration of themounting tube 4 is also at a minimum at that junction. The mounting tube4 typically vibrates in an axisymmetric mode with nodes and antinodes ofvibration distributed along its length. At the frequency of the radialbending mode (RB0) of the die ring 2, a node of the mounting tubepreferably coincides with the junction of the mounting tube and the diering so that the amplitude of vibration is at a local minimum there.

1. A die, comprising: a generally cylindrical die ring comprising an endsurface and having a radial bending mode of vibration in which the endsurface oscillates between a concave and a convex state; and a mountingtube coaxial with the die ring and extending from the end surface of thedie ring; characterized in that the mounting tube joins the end surfaceof the die ring at a radius where the amplitude of the oscillation ofthe end surface is at a minimum.
 2. The die according to claim 1,wherein the end surface is annular.
 3. The die according to claim 1,wherein, at the frequency of the radial bending mode of the die ring,the mounting tube vibrates in a mode in which the amplitude of vibrationis a local minimum at the junction of the mounting tube and the diering.
 4. A method of operating a die that comprises a generallycylindrical die ring having an end surface, and a mounting tube coaxialwith the die ring and extending from the end surface, the methodcomprising vibrating the die ring in a radial bending mode, in which theend surface of the die ring oscillates between a concave and a convexstate, characterized in that the minimum amplitude of the oscillation ofthe end surface occurs at a radius where the mounting tube joins the endsurface.
 5. The die according to claim 2, wherein, at the frequency ofthe radial bending mode of the die ring, the mounting tube vibrates in amode in which the amplitude of vibration is a local minimum at thejunction of the mounting tube and the die ring.